On 09/06/2016 14:16, Peter Zijlstra wrote:
> On Sat, May 28, 2016 at 08:27:43PM +0800, Minfei Huang wrote:
>> +++ b/arch/x86/kernel/pvclock.c
>> @@ -61,11 +61,14 @@ void pvclock_resume(void)
>> u8 pvclock_read_flags(struct pvclock_vcpu_time_info *src)
>> {
>> unsigned version;
>> - cycle_t ret;
>> u8 flags;
>>
>> do {
>> - version = __pvclock_read_cycles(src, &ret, &flags);
>> + version = src->version;
>> + /* Make the latest version visible */
>> + smp_rmb();
>> +
>> + flags = src->flags;
>
> Using a seqcount to load a single byte is insane ;-)
Only if you know that the writer will not write that byte twice within a
critical section...
Which I guess we do know in this case because the write side is just a
memcpy, but it's still a bit safer when it's not specified by the
pvclock API. It's not a fast path anyway, it runs literally twice at
startup.
Paolo
